Dinosaurs! They're big, heavy and spiky in all the wrong places. How in the hell could they have sex to reproduce? Smithsonian Magazine tackled this topic saying:

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Figuring out how Stegosaurus even could have mated is a prickly subject. Females were just as well-armored as males, and it is unlikely that males mounted the females from the back. A different technique was necessary. Perhaps they angled so that they faced belly to belly, some have guessed, or maybe, as suggested by Timothy Isles in a recent paper, males faced away from standing females and backed up (a rather tricky maneuver!). The simplest technique yet proposed is that the female lay down on her side and the male approached standing up, thereby avoiding all those plates and spikes.

Dinosaur style is pretty impressive! More detailed looks at dinosaur fossils showed that a lot of dinosaurs gave birth rather young and before they were fully mature. Some other dinosaurs had odd rituals in courtship like entwining necks. Read the rest of the interesting, to say the least, article at Smithsonian Mag. [Smithsonian Mag via Neatorama]